Present day reamers must be self-centering when the cut is initiated, in spite of the relatively large conatilever length inherent in their use. For reamers utilizing a plurality of bits, all having cutting points lying in the same plane when viewed in the axial direction, so that all bits commence to cut simultaneously, the centering requirement is automatically fulfilled.
In single bit reamers having a rectangular turnplate, the cut-initiating angles of the bit correspond to the initiating angles of the guide members even for two leading angles.
A single bit reamer with a bit having a circular cutting edge is disclosed in German Patent No. 16 27 221. The reamer has two guide members arranged symmetrically to the central axis of the cutter head. In this reamer, the bit and the guide members are arranged on a carrier which is movable within the body of the tool in a direction perpendicular to its axis. The bit and the two guide members provide three points of contact between the cutter head and the workpiece, automatically adjusting the position of the tool to be concentric to its axis of rotation. The resulting bore is therefore concentric to the axis of rotation. However, neither a precise diameter nor an exact concentricity (deviation less than approximately 5 microns) can be achieved with this type of reamer.